1) the excellent quality of construction
2) ease of servicing and use
3) designed to run 24/7 for years
4) no frills, but good solid, reliable hardware
5) Baseboard (BMC) management features
5) amazingly power frugal with the right settings

These servers were (reportedly) part of a large run of 10,000 or more units manufactured for cloud/cluster computing. Many of them have turned up in the surplus market over the past few years. The units I obtained had sequential serial numbers and were configured identically. They were very clean and in excellent condition. The BMC firmware was the latest available version, and the motherboard firmware was the penultimate version. I therefore chose not to attempt any firmware upgrades.